The invention relates to torquer apparatus.
An accepted way to control the attitude of a vehicle in space is the use of reaction wheels or control moment gyroscopes. Reaction-gyro spheres have also been proposed (see, for instance, U.S. Pat. No. 4,611,863), but are not available as commercial products.
The three approaches are based on the principle that the total momentum of a cluster of mobile bodies is constant when no external torque is applied. Thus, a change of the momentum of one body, e.g., a wheel or a sphere, implies an opposite change of the momentum of the other bodies, e.g., the vehicle. As the momentum is the product of the moment of inertia with the angular velocity, a change of momentum results in a change of angular velocity. The attitude of the vehicle can therefore be controlled using these approaches, despite changes of momentum caused by moving parts of instruments, and despite external torques, caused, for example, by the Earth's magnetic field or the solar light pressure. In some situations, the external torques may eventually significantly alter the total momentum, forcing the wheels or the sphere to spin at their maximum angular velocity. External forces applied, for example, by thrusters are then used for ‘de-saturation’, bringing back the total momentum to an acceptable value.
Reaction wheels are wheels with controlled angular velocities. Usually three or more wheels are used with their axes spanning a 3D space. The wheel axes are fixed to the vehicle. Therefore, for a constant angular velocity of the vehicle, these axes change their orientation and, for the total momentum vector to remain constant in magnitude and direction, the wheels must continuously exchange their momentum.
Control moment gyroscopes are rapidly spinning wheels mounted on actuated gimbals. To change the angular velocity of the vehicle, the spin axis has to be tilted (remember that the total momentum is constant) by applying torques on the actuated joints of the gimbals. For a constant angular velocity of the vehicle, the joints of the gimbals are not actuated (assuming that the mass of the gimbals is negligible). Some rotations are impossible with a single control moment gyroscope.
The ultimate mechanism for controlling the vehicle momentum is a sphere—or at least a body with sufficient symmetries to have a scalar moment of inertia—with means to actuate the rotation of the sphere about any axis. Such a sphere replaces three or more reaction wheels and does not need to internally exchange momentum. Unlike control moment gyroscopes, a sphere does not have to, but can also, spin. Moreover, any rotation is possible with a single sphere, and any rotation sees similar conditions (isotropy).
The bearing of the sphere could be gimbals. In this case, if they are actuated, and if the sphere initially spins, the system is a control moment gyroscope. The joints of the gimbals can also be free with the sphere actuated by means of a pseudo-spherical motor such as disclosed in U.S. Pat. No. 5,476,018 (the motor described in this patent is here qualified as ‘pseudo-spherical’ because the binding of the sphere with the gimbals introduces poles, i.e., singularities that break the spherical symmetry).
Alternatively, the bearing of the sphere can be a reluctant magnetic bearing such as disclosed in U.S. Pat. Nos. 4,611,863 or 4,961,352. In this case, as there is no contact with the sphere, to be actuated, the sphere has to become the rotor of a 3D motor. No contact with the sphere is advantageous, because it means lower vibrations, higher reliability, and longer lifetime. Three orthogonal pairs of arced linear inductive motors (U.S. Pat. No. 4,611,863) or three orthogonal pairs of ring inductive motors (U.S. Pat. No. 4,961,352) are used to control the bearing and the rotation of the rotor. Even though elegant, such motors are unfortunately highly inefficient, which is a problem of important concern in space. The principle of inductive motor is to drag the rotor thanks to a moving field exerting a ‘magnetic friction’. As a consequence, the magnetic friction checks any motions of the rotor that are not following the direction of the moving field. In other words, the rotation created by one pair of inductive motors is checked by the two other pairs, dissipating thermal energy in the rotor.
Therefore, there is a need for an efficient reaction-gyro sphere with magnetic bearing.